This invention relates to a stereocontrolled total synthesis of the known antibiotic thienamycin (I). ##STR2## Starting from acetone dicarboxylate, the synthesis proceeds in a stereo-selective way via intermediates II and IIa. ##STR3## wherein R.sup.1 is a readily removable protecting group such as benzyl, .beta.,.beta.,.beta.,-trichloroethyl, methyl, ethyl, phenyl, t-butyl and the like.